The present invention generally relates to phosphor materials. More particularly, this invention relates to processes for producing rare earth-containing phosphor materials in the form of particles having a core-shell structure, wherein the shell has a lower rare earth metal content than the core.
Phosphors exhibit luminescence and are commonly used in fluorescent lamps, light emitting diodes (LEDs), and various other applications. Phosphor compositions typically comprise a host material doped with an activator that prolongs the emission time. A wide variety of phosphor compositions are known, including rare earth compounds and transition metal compounds doped with one or more rare earth activators.
Fluorescent lamps typically comprise a glass tube that serves as a transparent glass envelope enclosing a sealed discharge space that contains an inert gas and mercury vapor. The inner surface of the glass tube is coated with a layer containing a phosphor composition that is separated from the tube by an ultraviolet (UV) reflecting barrier layer of, for example, alumina or a halophosphor. The mercury vapor is ionized through the application of a current to produce radiation having primarily ultraviolet (UV) wavelengths, which in turn is absorbed by the phosphor composition, resulting in excitation of the phosphor composition to produce visible light that is emitted through the glass tube.
The spectral composition of a light source is typically gauged by its color rendering index (CRI), which is a measure of the degree to which the psycho-physical colors of objects illuminated by a light source conform to those of a reference illuminant under specified conditions. The color-rendering properties and emission output of fluorescent lamps can be improved through the use of phosphor layers containing a mixture of red, green and blue-emitting phosphors, which in combination produce illumination of that appears to be white. As a non-limiting example, phosphor layers have been employed that contain a controlled mixture of europium-activated barium magnesium aluminate phosphor (BaMgAl10O17:Eu2+; BAM) as a blue-emitting phosphor, cerium- and terbium-coactivated lanthanum phosphate phosphor (LaPO4:Ce3+,Tb3+; LAP) as a green-emitting phosphor, and europium-activated yttrium oxide phosphor (Y2O3:Eu3+; YOE) as a red-emitting phosphor, mixed in appropriate ratios. As known in the art, the term Aactivated@ refers to the effect that doping with europium, cerium, terbium, and other dopants have with respect to the luminescence of a phosphor.
Terbium is currently used in all high-performance green phosphors, for example, (La,Ce,Tb)PO4:Ce3+,Tb3+ (LAP), (Ce,Tb)MgAl11O19:Ce3+,Tb3+ (CAT), and GdMgB5O10:Ce3+,Tb3+ (CBT). However, rare earth elements, and particularly terbium and europium, are relatively expensive metals and their prices impact the cost of phosphor compositions. As such, terbium currently is a significant impact on the cost of green phosphor compositions, and it would be desirable to minimize the cost impact that terbium and potentially other rare earth metals have on green phosphors and lighting systems that employ them.